1. Field of the Invention
This invention relates in general to spindle motors for use in magnetic disk storage systems, and more particularly to a method and apparatus for providing magnetic wear debris collection for an air bearing of a spindle motor in a hard disk drive.
2. Description of Related Art
The storage capacity and general performance of hard disk drives have improved steadily over the last decade in response to the increasing demands of the computer industry. These developments have been mainly fueled by corresponding advances in the components of magnetic recording technology and in electronic sophistication and miniaturization. In particular, magnetic read/write recording heads and disks are capable of supporting areal data storage densities typically ten times greater than they could ten years ago. This is being achieved by higher track densities, higher linear flux transition densities and lower recording head flying heights.
Data storage systems, such as disk drives, commonly make use of rotating storage disks. The storage disks are commonly magnetic disks but could also be optical. In a typical magnetic disk drive, a magnetic disk rotates at high speed and a transducing head uses air pressure to “fly” over the top surface of the disk. The transducing head records information on the disk surface by impressing a magnetic field on the disk. Information is read back using the head by detecting magnetization of the disk surface. The magnetic disk surface is divided in a plurality of concentric tracks. By moving the transducing head radially across the surface of the disk, the transducing head can read information from or write information to different tracks of the magnetic disk.
The recording medium, i.e., magnetic disk, holds information encoded in the form of magnetic transitions. The information capacity, or areal density, of the medium is determined by the transducer's ability to sense and write distinguishable transitions. An important factor affecting areal density is the distance between the transducer and the recording surface, referred to as the fly height. It is desirable to fly the transducer very close to the medium to enhance transition detection. Some fly height stability is achieved with proper suspension loading and by shaping the air bearing slider surface (ABS) for desirable aerodynamic characteristics.
Spindle motors are commonly used to rotate magnetic disks at high speeds. Frequently, conventional spindle motors comprise small electric motors equipped with standard ball bearings. However, electric motors having ball bearings are known to experience problems such as runout or vibration that can prevent information from being accessed from disks rotated by the motors. This is especially true, as advancements in data storage technology have increased magnetic disk storage densities.
The point is now being reached, however, where further advances are likely to be impeded by mechanical constraints. In particular, limitations will result from the spindle hub holding the stack of disks on account of the properties of the ball bearing assemblies used for rotational support. Random vibrations in both axial and radial axes ultimately lead to limitations in positioning accuracies and signal-to-noise ratios with consequences for storage capacities. The radial accuracy directly limits the number of concentric tracks on a disk on which data can be stored and reliably retrieved.
To overcome the problems associated with ball bearing electric motors, some disk drive systems now make use of electric motors having fluid hydrodynamic bearings. Bearings of this type are shown in U.S. Pat. No. 5,427,546 to Hensel, U.S. Pat. No. 5,516,212 to Titcomb and U.S. Pat. No. 5,707,154 to Ichiyama.
An exemplary hydrodynamic bearing typically includes a stationary shaft on which is mounted a rotary hub to which magnetic disks can be secured. There is no direct contact between the rotating hub and the shaft. Instead, a lubricating fluid such as air or oil forms a hydrodynamic bearing between the shaft and the rotary hub. Hydrodynamic pressure or pumping is frequently provided by a pattern of grooves, commonly in a herringbone configuration, defined either by the exterior surface of the shaft or the interior surface of the rotary hub. During rotation of the hub, the pattern of grooves provides sufficient hydrodynamic pressure to cause the lubricating fluid to act as a bearing between the shaft and the rotary hub. Frequently, capillary seals are used to retain the bearing fluid between the shaft and the rotary hub.
Still, oil bearings have some disadvantages. For example, oil bearings consume more power than ball bearings or air bearings. Furthermore, when oil bearings are used in the journal bearing environment, oil leakage can be problematic.
When used in association with spindle motors, air bearings provide numerous advantages. For example, air bearings are more efficient and consume less power than either ball bearings or oil bearings. Also, air bearings are quiet and have excellent run out characteristics. Air bearings also have disadvantages. For example, when air bearings are used in disk drive spindle motors, it can be difficult or expensive to simultaneously provide both thrust (e.g. axial) and journal (e.g. radial) bearing support. Also, sliding friction associated with thrust operations during motor start-up and shutdown can create wear debris that reduces the efficiency of the motor. Additionally, air bearings often require more space than either ball or oil bearings thereby providing less space for the motor. Finally, air bearings are typically not effective for low rotational speed applications.
In the future, spindle motor disk rotation speeds will steadily increase. As disk rotation speeds increase, the problems associated with standard oil bearings, air bearings and ball bearings will become magnified. Increased disk recording density is another trend in the industry. The combination of increased disk rotation speeds and increased recording densities will require disk drives to operate with improved run out characteristics.
Still, because of the associated advantages discussed above, air bearings will be used more frequently as the rotating speed is increased. One of the more serious problems with air bearings is the problem of accumulated debris. It is necessary that the bearing lubricant, i.e., air, be maintained with a minimum amount of loose particles. Particles in the lubricant may originate from ambient environment or may be generated by contact between moving surfaces during operation. Generally, wear particles are generated in the bearing areas during motor start and stop. Such particles exhibit relatively jagged edges. Wear particles tend to generate when two members of differing hardnesses contact each other. Since bearings typically have a clearance of approximately 5 to 10 microns, particles in the lubricant may act as abrasive ingredients and cause accelerated wear of groove patterns in the bearing and eventually failure of the bearing.
Prior art approaches for providing particle traps in hydrodynamic bearings, such as oil bearings, have placed such traps in the recirculation port and at a location far away from the journal and thrust bearings. In addition, such particle traps have relied on global lubricant recirculation to move particles into the trap. However, these approaches are neither suited nor effective for controlling debris accumulation in air bearings.
It can be seen then that there is a need for a method and apparatus for providing magnetic wear debris collection for an air bearing of a spindle motor in a hard disk drive.